How to calculate grid%ph_2？

[ruikun]

Hello wrfhelp
Recently,i was learning how modes compute grid%ph_2, and i had some confusion about how PARAM_FIRST_SCALAR, num_3d_m, moist(i,kk+1,j,im) and moist(i,kk,j,P_QV) are defined in the source code, and how I should get or compute them？
Thanks!
Ruikun

kk = kte-1
k = kk+1

qtot=0.
DO im = PARAM_FIRST_SCALAR, num_3d_m
qtot = qtot + moist(i,kk,j,im)
ENDDO
qvf2 = 1./(1.+qtot)
qvf1 = qtot*qvf2

grid%p(i,kk,j) = - 0.5*(grid%Mu_2(i,j)+qvf1*grid%Mub(i,j))/grid%rdnw(kk)/qvf2
qvf = 1. + rvovrd*moist(i,kk,j,P_QV)
grid%alt(i,kk,j) = (r_d/p1000mb)*(grid%t_2(i,kk,j)+t0)*qvf&
*(((grid%p(i,kk,j)+grid%pb(i,kk,j))/p1000mb)**cvpm)
grid%al(i,kk,j) = grid%alt(i,kk,j) - grid%alb(i,kk,j)
grid%p_hyd(i,kk,j) = grid%p(i,kk,j) + grid%pb(i,kk,j)